This invention relates generally to the storage of a waveform by a digital oscilloscope, and in particular to a digital system for detecting and storing waveform envelopes or maximum signal deviations along a waveform.
Conventional digital oscilloscopes record amplitude vs. time waveforms by acquiring amplitude samples at equally-spaced time points along the time axis of the waveform and converting the samples to digital data for storage and subsequent display. The waveform memory typically is capable of storing one complete screen width, or frame, of information. Because memory space is finite, only a limited number of samples may be acquired, irrespective of sampling rate, conversion speed, or time base sweep rate. For example, for a 1-K memory and four bits of data per sample, only 256 samples are required to fill the memory. In terms of time intervals between samples, this means that for a sweep rate of one millisecond per division (10 divisions per frame), 40 microseconds elapse from the time one sample is acquired until the next sample is acquired. Even with more sophisticated and expensive systems having 4-K memories and eight bits of resolution, only 512 samples may be acquired per frame. The utility of one frame of information thus recorded is somewhat restricted, and may be misleading or even erroneous because phenomena occuring between samples is not recorded.
Often it is desired to record high frequency signals over a long time frame, or to record the envelope of a waveform, or to detect narrow transients which may occur on an otherwise low-frequency waveform such as a 1 microsecond spike over a 10 second frame requiring 10 million words (2500 times larger than available memory space). Previously, this information could only be observed on an analog oscilloscope or stored on a bistable storage cathode-ray tube.